VCAs are a fairly common element of almost any RF or microwave circuit. Their function is to change the amplitude of a signal based on some external signal, usually a voltage or current. A common use is the leveling of a signal so that both strong and weak signals can be adjusted in amplitude to provide a constant level signal to the next stage of the circuit. Another use is the balancing of multiple signal paths so they all have the same gain. A third use would be to use a VCA to control the gain of an amplifier over temperature by varying the control voltage based on a measurement of the ambient temperature. This last use is to counter undesired changes to the gain of the amplifier when the ambient temperature changes.
The vast majority of presently available VCAs include either diodes, transistors, or FETs (field effect transistors). These active devices have non-linear transfer characteristics which result in distortion to RF and microwave input signals. This causes additional and unwanted signals to be generated which are not present in the original signal. For example, suppose two people are transmitting a signal (from a cell phone, for instance) on two different frequencies at the same time. If the two signals were applied to a non-linear device, several additional signals would be generated that would be on frequencies that are different from the original two frequencies. This is known as intermodulation distortion. These additional signals have the potential of causing interference to other services, like police or fire departments that use the same frequencies as the additional signals.
VCAs are designed to reduce intermodulation distortion to the smallest possible value, but due to the non-linear characteristics of the control devices used, there is no way to eliminate intermodulation distortion entirely. Therefore, there exists a real and present need for a VCA that can control the amplitude of an RF or microwave signal without generating any distortion products which result in intermodulation distortion.
U.S. Pat. No. 5,332,981, issued to Joseph B. Mazzochette, et al., issued Jul. 26, 1994, entitled “Temperature Variable Attenuator,” which is incorporated herein by reference, describes an attenuator that includes temperature variable resistors (thermistors) in the attenuating path. As shown in FIGS. 1A and 1B which are reproduced from FIGS. 1 and 3 of the '981 patent. conventional attenuators include a Tee attenuator 10 comprising a pair of identical series resistors R1 and a shunt resistor R2 and a Pi attenuator 12 comprising a series resistor R2 and two shunt resistors R1 and R3. FIG. 1 C is a plot reproduced from FIG. 2 of the '981 patent, showing a family of constant attenuation curves from 1 to 10 dB and a constant 50 ohm impedance curve descending from the upper left of the plot to the lower right. The vertical axis on this plot represents the value of shunt resistor R2 in the T attenuator 10 and the horizontal axis represents the values of series resistors R1. The point of intersection between the 50 ohm impedance curve and an attenuation curve gives the value of R1 and R2 that produce the attenuation represented by the attenuation curve and a 50 ohm impedance match.
In the temperature variable attenuator of the '981 patent, the temperature coefficient of resistance (TCR) of at least one resistor is different such that the attenuation of the attenuator changes at a controlled rate with changes in temperature while the impedance of the attenuator remains substantially constant. Thus, this device changes its attenuation based on the ambient temperature, but because it is constructed entirely of passive components it does not generate any intermodulation distortion. However, the attenuation of this device cannot be set to a predetermined value based upon a constant external voltage or current.
U.S. Pat. No. 5,999,064, issued to Robert Blacka, et al., issued Dec. 7, 1999, entitled “Heated Temperature Variable Attenuator,” which is also incorporated by reference, provides a heater in a temperature variable attenuator. The heater allows an external voltage or current to heat the thermistors that are part of the attenuating circuit to affect their resistance, and thus, the attenuation of the device. However, there are a number of limitations with this device which reduces its usefulness as a VCA.